Process for manufacturing synthetic inorganic silicates or the like



Patented May 11, 1954 PROCESS FOR MANUFACTURING SYN THETIC INORGANIC THELIKE SILICATES OR Clifiord Jones, Eccleston, St. Helens, England,assignor to Pilkington Brothers Limited, Lancashire, England, a Britishcompany No Drawing. Application June 1, 1950, Serial N0. 165,595

Claims.

This invention relates to the manufacture of a synthetic mullite.

Pure mullite is a very desirable refractory material because it consistsof closely interlocking crystals which give it good mechanical strength,and because it contains no glassy matrix liable to drain out of thematerial in service as a result of its fusibility being lowered byexternal chemical attack.

By the calcination at high temperature, usually above 1400 C., of Kaolin(A12Oe.2SiO2.2H2O), or of Kyanite or sillimam'te (which are twodifferent crystalline forms of AlzGaSlOz), or of other aluminiumsilicates, it is relatively easy to promote the growth of crystals ofmullite, the formula of which is approximately 3Al2O3.2SiO2: but in thematerial so produced there is a residue of excess silica (SiOz), usuallyin the form of a glassy matrix. This glassy matrix of silica isdetrimental in refractories for tank-furnaces used for the melting ofglass from the raw ingredients because, in the conditions existing insuch furnaces, it is liable to be attacked by alkaline dusts and furnacegases as a result of which its fusibility is lowered and it drains outof the refractory material leaving an open porous structure which maydisintegrate in service.

Thus; if a mullite refractory can be made in which there is no glassymatrix that can cause weakness by draining out of the structure in thepresence of alkaline dusts, etc., full advantage can be taken of thecharacteristically good mechanical and chemical properties of puremullite as a refractory for high temperature service.

Attempts to make an all-mullite refractory by introducing the correctquantity of alumina (A1203) to combine with this excess silica usuallyresult in a mixture of mullite, corundum (a particular crystalline formof A1203); and silica. For example, we have found that the calcinationof a mixture of crushed sillimanite (natural rock) and alumina inthe'proportions present in mullite results in a material containingmullite and corundum.

A. J. Bradley and A. L. Roussin (cf. Transactions of the CeramicSociety, (1932), 31, 422), have described mixtures of porcelains whichon X-ray examination gave spectra similar to but not identical with thatof mullite, and they suggested that the material should be calledPorzite.

H. P. Rooksby and J. H. Partridge, (cf. Journal of the Society of GlassTechnology, (1939), 23, 338), have described results of the X-rayexamination of the structures of artificial and natural mullite andsillimanite. They discounted the suggestion of Bradley and Roussin thattheir Porzite was a new aluminium silicate, and they identified threeforms of mullite, viz:

1. 0c mullite.-This contains 72% of A1203 and 28% of SiOz by weight; itsformula is This form was not identified in any of the natural samples,but it was prepared synthetically by grinding together pure silica(S102) and pure alumina (A1203) in the correct proportions and calciningthe mixture at 1750". There is no disclosure of the material beingmoulded into shaped articles.

2. {3 mulZite.This contains 72% to 78% of A1203 and 28% to 22% of SiOzby weight: its formula is approximately 3A12O32Si02.

The X-ray spectrum for this material was found to correspond to thatfound for mullite crystals in electrically fused (electro-cast)tankfurnace blocks, and to that for natural mullite from North America.A substance very similar crystallographically was prepared artificiallyby mixing pure silica (SiOz) with pure alumina (A1203) in excess of the72 to 28 alumina-tosilica ratio and calcining the mixture at l800.

3. 'y mullite-This was found in samples of natural material from theIsle of Mull (hence the name mullite). A material very similar to it wasprepared by repeating the process described in 1. above for thesynthesis of a mullite, but replacing 1% of the alumina (A1203) by 1% oftitania (T102) or by less than 1% of ferric oxide (Fe2o3).

Accordingly, the present invention provides a process for themanufacture of synthetic mullite wherein an intimate mixture of alumina.and aluminium silicate bonded by means of an ethyl silicate is calcinedat a temperature of at least l500 0., the proportions of the ingredientsbeing such that the ratio of alumina to silica in the calcined mixtureis between 72% and 78% by weight of A1203 and between 28% and 22% byweight of SiOz.

The calcination temperature should be at least 1500", but a highertemperature, e. g. of 1650 to 1700 C. may be attained when it isrequired to produce a very dense material.

The aluminium silicate (such as Kyanite or sillimanite) should be ofhigh purity and both the aluminium silicate and the alumina must befinely ground (i. e. must pass through a 200 mesh sieve or finer).

Preferably, the binding agent is an ethyl silicate or mixture of ethylsilicates, such as the material sold as silicon ester and usuallycontaining 40 to 47% by weight of silica (SiOz). The silicon ester mayconveniently be used in solution in water or as a dispersion in analcohol-water mixture, and the quantity of silicon ester thus introducedinto the mixture can be adjusted according to the consistency requiredfor consolidating the final mix, as little as 1% by weight of the drymixture may be appropriate in some cases, and experiments have shownthat a quantity of 5% by weight of the dry mixture has produced veryeflicient results.

The ingredients should be very thoroughly mixed. When the silicon esterbreaks down, the pure silica produced is in a very finely divided stateand is more reactive than ordinary fineground silica. Since this silicais introduced as a solution or dispersion it may be assumed to be veryuniformly distributed.

The following example illustrates how the process of the invention maybe carried into eiiect:

500 gms. of a natural rock sillimanite containing 58% (:290 gms.) ofA1203 and 39% (:195 gms.) of S102 crushed to 200-mesh were intimatelymixed with 500 gms. of calcined alumina, A1203, crushed to ZOO-mesh and247 c. c. of silicon ester (containing 46.5% by weight of silicaequivalent to 112 gms. of $102) in an alcohol-water so- ,lutionconsisting of 60 c. c. of alcohol and 27 c. c.

of water. Thus the ratio of total alumina to total silica in the mixturewas 72 to 28. The mixture, after shaping with a required article, wascalcined at between 1650 C. and 1700 C. by heating in a gas-air firedfurnace. The product was a synthetic amullite.

By slightly increasing the ratio of alumina to silica pmullite may beprepared in a similar manner.

In the manufacture of refractories intended for use in the making ofglass, experience has shown that the aluminium compound herein describedshould be calcined at a temperature of at least 1500 C.

The mixture is eminently suitable for pressing, hammering, jolting orslip-casting to the required shape before calcining. The moisturecontent appropriate to the shaping process used is obtained byadjustment of the quantity of water the mixture and by allowing themixture to dry out if necessaryit hardens fairly quickly in warm air.

The porosity of the material produced can be reduced by crushing aftercalcining, then re-bonding with an organic binder such as polyvinylalcohol, and shaping and recalcining.

A synthetic mullite manufactured according to the present invention maybe used as a bonding material for certain other refractories, forexample corundum, but notwith materials such as zirconium with which themullite would react to lower the melting point of the mixture.

I claim:

1. A process for the manufacture of a synthetic substitute for naturalmullite which comprises calcining an intimate mixture of alumina,aluminium silicate and ethyl silicate as a binding agent, at atemperature of between 1500 C. and 1800" C., the initial proportions ofthe ingredients being such that in the calcined mixture the ratio ofalumina to silica is between 72% and 78% by weight of A1203 and between28% and 22% by weight of SiOz.

2. A process for the manufacture of a synthetic substitute for naturalmullite which comprises calcining an intimate mixture of alumina,aluminium silicate and an ethyl silicate containing between 40% and 47%by weight of S102 as a binding agent, at a temperature of between 1509C. and 1800 C., the initial proportions of the ingredients being suchthat in the calcined mixture the ratio of alumina to silica is between72% and 7 8 by weight of A: and between 28% and 22% by weight of S102.

3. A process for the manufacture of a synthetic substitute for naturalmullite which comprises calcining an intimate mixture of alumina,aluminium silicate and an ethyl silicate as a binding agent, at atemperature of between 1650 and 1720 C., the initial proportions of theingredients being such that in the calcined mixture the ratio of aluminato silica is between 72% and 78% by weight of A1203 and between 28% and22% by weight of S102.

A process for the manufacture of a synthetic substitute for naturalmullite which comprises intimately mixing together alumina, aluminiumsilicate and an aqueous alcoholic solution of an ethyl silicate,calcining the mixture thus procluced at a temperature of between 1500 C.and 1800 C., the initial proportions of the ingredients being such thatin the calcined mixture the ratio of alumina to silica is between 72%and 78 by weight of A1203 and between 28% and 22% by weight al SiOz.

5. A process for the manufacture of a synthetic substitute for naturalmullite which comprises intimately mixing together alumina, sillimaniteand an aqueous alcoholic solution of an ethyl silicate, calcining themixture thus produced at a temperature of between 1650 and 1700 C., theinitial proportions of the ingredients being such that in the calcinedmixture the ratio of alumina to silica is between 72% and 78% by weightof A1203 and between 28% and 22% by weight of SiO2.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,450,327 Cogan et a1 Sept. 28, 1948 FOREIGN PATENTS NumberCountry Date 957,817 France 1949 242,455 Germany 1912

1. A PROCESS FOR THE MANUFACTURE OF A SYNTHETIC SUBSTITUTE FOR NATURALMULLITE WHICH COMPRISES CALCINING AN INTIMATE MIXTURE OF ALUMINA,ALUMINIUM SILICATE AND ETHYL SILICATE AS A BINDING AGENT, AT ATEMPERATURE OF BETWEEN 1500* C. AND 1800* C., THE INITIAL PROPORTIONS OFTHE INGREDIENTS BEING SUCH THAT IN THE CALCINED MIXTURE THE RATIO OFALUMINA TO SILICA IS BETWEN 72% AND 78% BY WEIGHT OF AL2O3 AND BETWEEN28% AND 22% BY WEIGHT OF SIO2.